Printed circuit board capacitor structure and method

ABSTRACT

A capacitive element for a circuit board or chip carrier having improved capacitance and method of manufacturing the same is provided. The structure is formed from a pair of conductive sheets having a dielectric component laminated therebetween. The dielectric component is formed of two or more dielectric sheets at least one of which can be partially cured or softened followed by being fully cured or hardened. The lamination takes place by laminating a partially cured or softened sheet to at least one other sheet of dielectric material and one of the sheets of conductive material. The total thickness of the two sheets of the dielectric component does not exceed about 4 mils and preferably does not exceed about 3 mils; thus the single dielectric sheets does not exceed about 2 mils and preferably does not exceed about 1.5 mils in thickness. The use of two or more sheets of dielectric material makes it very unlikely that two or more defects in the sheets of dielectric material will align thus greatly reducing the probability of a defect causing a failure in test or field use.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to capacitive elements for circuitboards having improved decoupling capacitance reliability, and moreparticularly to a circuit board or chip carrier or the like and methodof manufacturing the same wherein the board or chip carrier usesmultiple layers of dielectric material yet achieves high capacitance.

2. Background Art

In the manufacture of circuit boards or chip carriers or the like it isdesirable to create as high a capacitance as possible between variousground, voltage and signal planes to thereby minimize the amount ofdiscrete decoupling capacitors required on the board surface. Theequation for determining capacitance is C=eA/t where C is thecapacitance, e is the dielectric constant or relative permittivity ofthe dielectric material, A is the available area, and t is the thicknessof the dielectric material or the spacing between the plates of thecapacitor. Thus to increase the capacitance of a capacitor of a givenarea one can select a material having an increased dielectric constantand/or decrease the thickness of the dielectric material. However, theselection of the dielectric material is often limited by many productionand configuration limitations which leaves principal means of increasingthe capacitance to a reduction in thickness of the dielectric material.However, a problem encountered with certain types of dielectric materialespecially epoxy impregnated glass cloth, but other materials as well,at thinner gauges is that any defects in the material in the form of pinholes or voids tend to extend from one surface to the other thusresulting in failure of the structure either under test conditions, oreven worse they may pass testing but fail under field use conditions.Therefore it is desirable to form a circuit board or chip carrier havingreduced thickness of the dielectric material for improved capacitancebut with improved reliability.

SUMMARY OF THE INVENTION

According to the present invention a capacitive element for a circuitboard or chip carrier having improved capacitance and method ofmanufacturing the same is provided. The structure is formed by selectinga pair of conductive sheets and laminating a dielectric componentbetween the two sheets. The dielectric component is formed of two ormore separate sheets of dielectric material at least one of which can bepartially cured or softened followed by being fully cured or hardened.The lamination takes place by laminating a partially cured or softenedsheet to at least one other sheet of dielectric material and one of thesheets of conductive material. The total thickness of the two sheets ofthe dielectric component should not exceed about 4 mils and preferablyshould not exceed about 3 mils; thus the single dielectric sheets shouldnot exceed about 2 mils and preferably should not exceed about 1.5 milsin thickness. The use of two or more sheets of dielectric material makesit very unlikely that two or more defects in the sheets of dielectricmaterial will align thus greatly reducing the probability of a defectcausing a failure in test or field use.

In one embodiment a pair of ultra thin epoxy impregnated sheets of glasscloth are B-stage (partially) cured and then are laminated between twosheets of conducting material, preferably copper, with the laminationprocess fully curing the epoxy impregnated cloth to form the laminatestructure. In other embodiments one or more sheets of dielectricmaterial are laminated to one or more sheets of conductive material or afilm of dielectric material is coated on one or more sheets ofconductive material and B-stage cured or softened and then furtherlaminated and fully cured or hardened to form the structure. In otherembodiments sheets of various dielectric materials are laminated to eachother and to copper sheets by various techniques. In still otherembodiments films of dielectric material are coated on copper sheets andB-stage cured, laminated and fully cured. In yet other embodimentsdifferent dielectric materials are used to form the dielectriccomponent; for example epoxy is first coated on opposite sides ofanother dielectric material such as a polyimide sheet, B-stage cured,and then further laminated to two sheets of conducting material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a side elevational view of the components of a circuit boardassembled for lamination according to one embodiment of this invention;

FIG. 1b is the components shown in FIG. 1a laminated together to form acircuit board element;

FIG. 1c is the board of FIG. 1b prepared for etching the copper sheetsto form clearance holes;

FIG. 1d is the board of FIG. 1c with the copper sheets etched to formthe clearance holes;

FIG. 2a is a side elevational view of the components of a circuit boardassembled for lamination according to another embodiment of thisinvention;

FIG. 2b is the components of FIG. 2a laminated to form a circuit boardelement;

FIG. 3a is a side elevational of the first step in forming a circuitboard according to yet another embodiment of this invention;

FIG. 3b depicts the step following that shown in FIG. 3a in themanufacture of the circuit board element;

FIG. 3c depicts the final lamination step after that shown in FIG. 3b inthe manufacture of the circuit board element;

FIG. 4a is a side elevational view showing the components ready forlamination of still a further embodiment of this invention;

FIG. 4b shows the lamination of the components depicted in FIG. 4a;

FIG. 5a is a side elevational view depicting the structure of onecomponent of a circuit board according to still another embodiment ofthis invention;

FIG. 5b depicts the laminate structure of a circuit board using thecomponent of FIG. 5a; and

FIG. 6 is a longitunal sectional view of two capacitive elements used toform a completed circuit board.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and for the present to FIGS. 1a and 1 bone embodiment of the present invention is shown. FIG. 1a depicts alay-up of the components of a circuit board according to one embodimentof this invention which components include a pair of conducting sheetsof material 10, preferably copper. The copper can be either preferably ½oz. or 1 oz. copper sheets, although thicker or thinner copper sheetscould be used, suitable sheets being available from Gould Corp. otherconductive material such as, but not limited to copper-Invar-copper, orcopper on an aluminum carrier can be used. The sheets preferably haveone surface 11 roughened to improve adhesion to other materials as iswell known in the art. A pair of sheets 12 a of dielectric material areprovided, located between the copper sheets 10. The dielectric sheetsare preferably ultra thin sheets of glass cloth which have beenimpregnated with an epoxy and partially (B-stage) cured. This B-stagecuring is accomplished by heating to about 100° C. for about 5 to about20 minutes. Preferably the cloths are 101 or 104 style glass clothsmanufactured by Clark-Schwebel Inc. The 101 style cloth is about 1.0 toabout 1.1 mil thick and the 104 glass cloth is about 1.5 mils thick. Theepoxy resin preferably can be any one of several resins such as aphenolically hardened epoxy resin. Glass cloths impregnated with thistype of resin are sold by IBM under the trade mark Driclad™. (As usedherein the dielectric resinous material in its partially cured orsoftened stage is designated by the number eg “12” followed by theletter “a” thus “12 a”, and in the fully cured or hardened state by thenumber e.g. “12” followed by the letter “b” thus “12 b”.) The sheets ofcopper 10 and the sheets of dielectric material 12 a are then laminatedtogether in a laminating press at a temperature of from about 180° C. toabout 200° C. at between about 300 psi to about 500 psi for about 1 to 2hours. This will result not only in laminating the components but alsoin fully curing the impregnated B-stage cured cloth 12 a resulting in apair of fully cured glass cloths 12 b laminated between a pair of coppersheets 10 to thus form the basis of a circuit board or chip carrier 14as shown in FIG. 1b. The thickness of the two glass cloths 12 b will beless than about 4 mils, and if style 101 cloth is used the thicknesswill be less than about 3 mils.

As indicated earlier one advantage of using two sheets of impregnatedglass cloth is to greatly reduce the potential for failure of thestructure due to defects such as pin holing in the manufacturing of theepoxy impregnated glass cloth. If a defect were to occur in one of thecloths it is highly unlikely that a similar defect would appear in theother cloth aligned or coincident with the defect in the first cloth;hence there is much less likelihood of failure of the part under testingor field conditions.

The laminate structure 14 forming the capacitive element can be furtherprocessed into a completed circuit board by any of a number of processeswell known in the art. For example the copper sheets 10 may be used asvoltage planes, e.g. a power plane and a ground plane. In such caseclearance holes need to be provided in the copper sheets 10. This can beaccomplished, for example, by photolithography and subtractive etching,as shown in FIGS. 1c and 1 d.

Referring to FIG. 1c both of the copper sheets 10 are coated with aphotoresist 16; and, using photolithography the photoresist is exposedand developed to reveal the underlying copper sheets 10 at the locationthat the clearance holes are to be formed. The revealed copper is thenetched with conventional etchant to form holes 18 and the remainingphotoresist stripped as seen in FIG. 1d. This structure can then belaminated with other structures, or be used as a base on which to formsignal planes or connections to produce a completed circuit board orchip carrier using well known techniques.

Referring to FIGS. 2a and 2 b another embodiment of the presentinvention is shown. In this embodiment a pair of copper sheets 10 arecoated each on one side thereof with a dielectric material 13 a. Thematerial may be epoxy of the type previously described, or other typesof dielectric material could be used, such as, a cyanate ester, apolyimide, or polytetraf luoroethlyene (PTFE). The dielectric materials,other than the impregnated glass cloth may be applied as liquids or inthe case of polyimide and PTFE be in the form of free standing films ofmaterial. The material is partially cured as described previously, or inthe case of films or glass cloth may be applied to the copper in thepartially cured form, and the sheets of copper 10 with the dielectricmaterial 13 a thereon are laminated together to form structure 20, shownin FIG. 2b comprised of two sheets of copper 10 separated by two sheetsof fully cured dielectric material 13 b. The total thickness of the twodielectric layers 13 b together should not exceed about 4 mils andpreferably should not exceed about 3 mils. The structure can be furtherprocessed into a circuit board or chip carrier as previously described.The term partially cured and fully cured material is used herein todenote specifically the curing of thermoset resins by cross linking, thepartial cure referring to less than complete cross linking and full curereferring to a more complete cross linking. (The terms partial cure andfull cure may also sometimes be used herein in referring tothermoplastics. In such case heating of the thermoplastic causes asoftening of the material which sometimes is referred to herein aspartial cure and cooling causes the material to harden sometimesreferred to herein as complete cure; although sometimes the moreappropriate terms “softened” and “hardened” are used when referringspecifically to thermoplastics.)

Referring to FIGS. 3a-3 c yet another embodiment of the invention isshown. In this embodiment a layer of dielectric material 13 b such as anepoxy is applied to a copper sheet 10 and fully cured to form thecomponent structure 22 as shown in FIG. 3a. Following this a layer ofdielectric material 13 a preferably also an epoxy is applied to thelayer 13 b of dielectric material and partially cured to form componentstructure 24 as shown in FIG. 3b. Thereafter a sheet of copper 10 islaminated to the layer of dielectric material 13 a, the lamination fullycuring the layer 13 a of FIG. 3b to form the composite structure 26shown in FIG. 3c of two sheets of dielectric material 13 b laminatedbetween two sheets of copper 10.

Referring now to FIGS. 4a and 4 b still another embodiment of theinvention is shown. FIG. 4a shows the components ready to be laminated,which components include a sheet of copper 10 having coated thereon acoating of epoxy 13 b which is fully cured. (It should be noted that theepoxy 13 b could be only partially cured if desired.) A sheet of glasscloth impregnated with epoxy resin and partially cured 12 a is providedas well as a second sheet of copper 10. (The second sheet of copper 10can optionally be coated with a dielectric material also.) Thecomponents of FIG. 4a are then laminated to form the structure 28 shownin FIG. 4b. This embodiment illustrates how different forms of the epoxycan be used.

Referring to FIGS. 5a and 5 b still another embodiment of the inventionis shown which depicts one way in which several different dielectricmaterials are combined. As shown in FIG. 5a a sheet of polyimide 30 iscoated on each side with a layer of epoxy 13 a which epoxy is partiallycured to form a dielectric component 32. A pair of copper sheets 10 arelaminated one to each of the layers of epoxy and as described previouslythe epoxy fully cures during lamination to form composite structure 34of a dielectric material formed of a sheet of polyimide 30 and twolayers of fully cured epoxy 13 b sandwiched between a pair of coppersheets 10 as shown in FIG. 5b.

In the embodiments shown the capacitance, according to the formula givenabove, should be at least about 500 pico farads per square inch forelements having a dielectric thickness of about 2 mils, and at leastabout 250 pico farads per square inch for elements having a thickness ofabout 4 mils.

It should be noted that in all of the embodiments shown one or more ofthe layers of dielectric material may be filled with particulate matterhaving a high dielectric constants, such as for example, but withoutlimitation, barium titanate, strontium titanate, lead-zirconiumtitanate, and tantalum oxide. The particle size can range from about 0.1micron up to about one-half the thickness of the layer containing theparticulate material. It is preferred that there be a range of sizes ofthe particles to promote increased volumetric packing efficiency andhence an increased dielectric constant e. Such loading will increase thecapacitance somewhat.

FIG. 6 shows in cross section a circuit board utilizing two capacitiveelements of the type shown in FIG. 1D. It is to be understood, however,that other capacitive elements of this invention such as those shown inFIGS. 2B, 3C, 4B or 5B could also be utilized.

Shown in FIG. 6 are a pair of capacitive elements 14 of the type shownin FIG. 1D each having laminated to the copper sheet 10 on each sidethereof, a layer of dielectric material 40. A signal core comprised ofdielectric material 42 and circuitry 44 formed on both sides of adielectric material 42 is placed between a pair of capacitive elements14, and adjacent to dielectric material 40. Also circuitry 46 is formedon the exposed surfaces of each of the sheets of dielectric material 40.Plated through holes 48 are formed to connect the circuitry 44, 46 andappropriate copper sheets 10 which form voltage and power planes.

In a preferred technique for forming the circuit board a pair ofdielectric layers 42 with the circuitry 44 thereon are provided aspreformed signal planes, and the dielectric layers 40 are all providedas b stage cured sticker sheets of fiber glass impregnated with epoxy.These are stacked up with the capacitive elements and a sheet of copperfoil applied to each of the exposed surfaces of the sticker sheets 40where the circuitry 46 is to be formed. The stack is laminated togetherunder heat and pressure sufficient to fully cure the sticker sheets andbond all of the components together. The through holes are then drilledand the holes plated with copper. The surface of the copper foil is alsoplated. The external circuit features 46 are then defined by standardphoto lithographic and copper etch techniques. These processes to formthe circuit board are all well known in the art.

It also should be understood that additional capacitive elements andsignal planes could be employed, as well as the other configurations ofthe capacitive elements. Also other techniques well known in the artcould be employed to form the circuit board.

Accordingly, the preferred embodiments of the present invention havebeen described. With the foregoing description in mind, however, it isunderstood that this description is made only by way of example, thatthe invention is not limited to the particular embodiments describedherein, and that various rearrangements, modifications, andsubstitutions may be implemented without departing from the true spiritof the invention as hereinafter claimed.

What is claimed is:
 1. A capacitive element, comprising: at least twosheets of thermoplastic or thermosetting resinous dielectric material,each sheet containing at least one resin impregnated sheet of glasscloth and having a thickness of between about 1 mil and about 2 mils,said sheets of dielectric material being bonded together to form adielectric component having a thickness of between about 2 mils andabout 4 mils; first and second sheets of conductive material; saiddielectric component being laminated between said first and secondsheets of conductive material and being fully cured or hardened.
 2. Thecapacitive element according to claim 1 wherein three sheets ofdielectric material are bonded together to form the dielectriccomponent.
 3. The capacitive element according to claim 1 wherein the atleast two sheets are formed from different materials.
 4. The inventionas defined in claim 1 wherein the sheets of cloth incorporate an epoxyresin which is fully cured.